Nanostructures Fabrication Methods
•bottom-up methods (“atom by atom”)
In the bottom-up approach, atoms, molecules and even nanoparticles themselves can be used as the building
blocks for the creation of complex nanostructures; the useful size of the building blocks depends on the
properties to be engineered. By altering the size of the building blocks, controlling their surface and internal
chemistry, and then controlling their organization and assembly, it is possible to engineer properties and
functionalities of the overall nanostructured solid or system. These processes are essentially highly
controlled, complex chemical syntheses.
•top-down processes (removal of reformation of atoms to create
the desired structure)
Top-down approaches are inherently simpler and rely either on the removal or division of bulk material, or on
the miniaturization of bulk fabrication processes to produce the desired structure with the appropriate
properties.
Top-down processes
Top-down processes are effectively examples of solid-state processing of materials
•milling
•lithographic processes
•machining
Milling
Mechanical attrition or mechanical
alloying
Microstructures and phases produced
in this way can often be
thermodynamically metastable
Lithographic processes
Conventional lithographic processes are akin to the emulsion-based photographic process and can be used to create
nanostructures by the formation of a pattern on a substrate via the creation of a resist on the substrate surface.
Visible or UV light
X-rays
Electrons or ions
to project an image containing the desired pattern onto a surface coated
with a photoresist material
The resist material, typically a polymer, metal halide or metal oxide, is chemically
changed during irradiation, often altering the solubility or composition of the
exposed resist.
Lithographic processes
Pattern transfer processes
•Solution-based wet chemical etching
procedures
•Dry etching in a reactive plasma (reactive
ion etching RIE, chemically assisted ion
beam etching CAIBE)
•Doping using ion implantation techniques
•Thin film deposition
Fundamentally, the wavelength of the radiation used in the lithographic process
determines the detail in the resist and hence the final planar nanostructure;
additional considerations may involve the limitations of the projection optics and
the nature of the interaction of the radiation with the resist material. Typically
the resolution ranges from a few hundred nanometres for optical techniques to
tens of nanometres for electron beam techniques. Phenomenologically,
throughput and resolution of lithographic techniques broadly follow a power-law
dependence; the resolution is approximately equal to 23A0.2 , where A is the
areal throughput.
Lithographic processes
Schematic representation of the photolithographic process
sequences, in which images in the mask are transferred to the
underlying substrate surface.
d
The theoretical resolution capability of shadow
photolithography with a mask consisting of equal
lines and spaces of width b due to diffraction is
given by:
s
=400 nm, d=1 μm, resolution slightly less than 1 μm
In contact-mode photolithography, the mask and wafer are
in intimate contact, and thus this method can transfer a
mask pattern into a photoresist with almost 100% accuracy
and provides the highest resolution. However, the maximum
resolution is seldom achieved because of dust on
substrates and non-uniformity of the thickness of the
photoresist and the substrate.
Such problems can be avoided in proximity printing, in
which, a gap between the mask and the wafer is introduced.
However, increasing the gap degrades the resolution by
expanding the penumbral region caused by diffraction.
In projection printing techniques, lens elements are used to focus the mask image onto a wafer substrate, which is
separated from the mask by many centimeters. Because of lens imperfections and diffraction considerations,
projection techniques generally have lower resolution capability than that provided by shadow printing
Lithographic processes
Deep Ultra-Violet lithography (DUV) - wavelengths below 300 nm
Technical challenges:
Lower output in DUV (10-20 watts, KrCl and KrF excimer lasers 222 nm and 249 nm)
With DUV, optical lithography allows one to obtain patterns with a minimal size of lOOnm
Extreme UV (EUV) lithography with wavelengths in the range of 11-13 nm has also been explored for
fabricating features with even smaller dimensions and is a strong candidate for achieving dimensions of
70nm and below.
Problems: refractive in this wavelength regime is very strong, and refractive lens can not be used.
Phase-shifting lithography
Lithographic processes
Parallel lines formed in photoresist using near field contact-mode photolithography have
widths on the order of 100 nm and are -300 nm in height as imaged by (A) AFM
and (B) SEM. [J.A. Rogers, K.E. Paul, R.J. Jackman, and G.M. Whitesides, .J Vac. Sci.
Technol. B16, 59 (1998).]
Lithographic processes
Electron beam lithography
Electron beams can be focused to a few nanometers in diameter and rapidly deflected either electromagnetically or
electrostatically. Electrons possess both particle and wave properties; however, their wavelength is on
the order of a few tenths of angstrom, and therefore their resolution is not limited by diffraction considerations.
Resolution of electron beam lithography is, however, limited by forward scattering of the electrons in the resist
layer and back scattering from the underlying substrate. Nevertheless, electron beam lithography is the most
powerhl tool for the fabrication of feathers as small as 3-5 nm.
Four typical subsystems:
(i)
Electron source (gun)
(ii)
Electron column (beam forming system)
(iii) Mechanical stage
(iv) Control computer
Lithographic processes
X-ray lithography
X-rays with wavelengths in the range of 0.04 to 0.5 nm represent another alternative radiation source
with potential for high-resolution pattern replication into polymeric resist materials
(a) 35 nm wide Au lines grown by
electroplating using a template fabricated by
X-ray lithography. The mean thickness is
about 450 nm, which corresponds to an
aspect ratio close to 13.
(b) 20 nm wide W dots obtained after
reactive ion etching of 1250nm
thick W layer.
[G. Simon, A.M. Haghiri-Gosnet, J. Bourneix,
D. Decanini, Y. Chen, F. Rousseaux, H. Launios,
and B. Vidal, .I Vac. Sci. Techno/. B15, 2489
(1997).]
Lithographic processes
Focused ion beam (FIB) lithography
FIB lithography is capable of producing electronic devices with submicrometer dimensions
Ions with energy in the MeV range, scattering is much more less
But: lower throughput, substrate damage
SEM image showing a regular array of 36 gold pillars in each
corresponding to an individual ion beam spot created using
chemical assisted FIB deposition.
[A. Wargner, J.P. Levin, J.L. Mauer, PG. Blauner, S.J. Kirch, and
P. Longo,J. Vuc. Sci. Technol. B8, 1557 (1990).]
Lithographic processes
Neutral atomic beam lithography
In neutral atomic beams, no space charge effects make the beam divergent; therefore, high kinetic particle
energies are not required. Diffraction is no severe limit for the resolution because the de Broglie wavelength of
thermal atoms is less than 1 angstrom. These atomic beam techniques rely either on direct patterning using
light forces on atoms that stick on the surface or on patterning of a special resist
Schematic illustrating the basic principles of neutral atom
lithography with light forces. [B. Brezger, Th. Schulze, U.
Drodofsky, J. Stuhler, S. Nowak, T. Pfau, and
J. Mlynek, J. Vac. Sci. Technol. B15, 2905 (1997).]
SEM image showing chromium nanowires of 64nm on
silicon substrate grown by neutral atomic beam
deposition with laser forces [ibid.]
Lithographic processes
Soft lithography techniques pattern a resist by physically deforming
(or embossing) the resist shape with a mould or stamp, rather than by
modifying the resist chemical structures with radiation as in
conventional lithography. Additionally the stamp may be coated with a
chemical that reacts with the resist solely at the edges of the stamp. These methods circumvent many of the resolution limitations inherent in conventional
lithographic processes that arise due to the diffraction limit of the radiation, the projection
or scanning optics, the scattering process and the chemistry within the resist material. Ultimately, nanoimprinting should represent a cheaper process for mass production.
Currently, these soft lithography techniques can produce patterned structures in the range
10 nm and above. One of the main limitations on resolution arises from plastic flow of the
polymeric materials involved. Master moulds may be fabricated using either conventional
lithographic techniques, micromachining or naturally occurring surface relief on the
substrate materials.
Machining
Lithographic techniques essentially consist of a two-dimensional chemical or mechanical patterning of the surface
of a material.
Three-dimensional patterning of a material can be achieved by techniques analogous to more conventional
machining.
Currently resolution limits are of the order 5 μm, but in recent year focused ion beams (FIB) and highintensity lasers have been used to directly pattern or shape materials at micron and submicron levels.
Scanning electron microscope image of a multilevel gear structure
created by focused ion beam sputtering of silicon